1. Field of the Invention
The present invention relates to a coating composition. The inventive composition can be applied to a substrate, particularly a plastic substrate for optical applications, to enhance the hardness of the substrate and prevent the substrate from being scratched.
2. Description of the Prior Art
Due to careless operations, the surfaces of substrates are easily be scratched or worn, which adversely affects the appearance and properties of the substrates. Particularly, optical substrates are often impaired by the vibration during transportation. Presently, the solution adopted in the industry is to adhere a protective film to the surface of a substrate. Nevertheless, the utilization of a protective film will increase the cost. To overcome the above drawback, a helpful approach would be to apply a coating to the surface of a substrate to enhance the hardness of the substrate and prevent the surface of the substrate from being scratched. In the past, UV curable resins were normally used as scratch-resistant coatings because they can react in a short time and achieve a degree of crosslinking. However, the utilization of UV curable resins is limited by insufficient UV light transmission, which may result in a non-cure or an incomplete cure. Moreover, due to a crosslinking reaction, UV curable resins are subject to inconsistent stresses or shrinkage rates and easy to warp or crack.
In order to address the problems associated with the UV light transmission and non-cure or incomplete cure in conventional photo-curing techniques, and due to the fact that thermally curable resins cure in a prolonged time, CN10106347A (US2007/0066698 A1) discloses a dual cure composition which comprises at least one filler, at least one curable monomer comprising at least one of an ethylenic unit or cyclic ether unit or mixture thereof, at least one photoinitiator; and at least one thermal initiator. CN10106347A discloses exposing the dual cure composition to radiation to at least partially photocure, and providing sufficient heat to initiate thermal curing so as to obtain a cured composition.
U.S. Pat. No. 5,571,297 also discloses a dual cure binder system to address the problems associated with conventionally used phenolic resin that has excellent adhesion property but needs a prolonged heating to achieve thermal curing, and to obviate the above-mentioned drawbacks associated with the photocure. U.S. Pat. No. 5,571,297 discloses a coated abrasive with a binder coat which comprises a compound having at least one function that is radiation curable and at least one function that is polymerizable under thermally activated conditions to achieve the desired effects.
Incorporating a certain amount of a thermally curable resin into a coating that contains a UV curable resin and utilize a dual cure process to effectively generate a coating with a low degree of crosslinking is already known in the art. For example, DE 19920799, U.S. Pat. Nos. 4,025,407, and 6,835,759 have disclosed such technique. Normally, the thermally curable resins used are thermal setting resins. These thermal setting resins have a great internal stress and cannot fully solve the problem associated with warping. In addition, the curing of a thermal setting resin normally requires a curing agent (or a crosslinker). However, curing agents react with the thermal setting resin easily, thereby increasing the molecular weight of the resin over time or resulting in different extents of reaction, and changing the properties of the resin. In this case, the resin should be consumed in a limited period and is not suitable for long-time coating.
U.S. Pat. No. 6,835,759 discloses a coating composition comprising a radiation curable component (a1), a thermally curable binder component (a2), a thermally curable crosslinking compound (a3), and optionally, one or more reactive diluents (a4) in a ratio that is obtained by the following equation: UV/TH=[a1+a4]NV/[a2+a3]NV. wherein [a1+a4]NV refers to the total nonvolatile weight of components (a1) and (a4). and [a2+a3]NV refers to the total nonvolatile weight of components (a2) and (a3). In a preferred embodiment, when UV/TH is from 0.25 to 0.50, most preferably from 0.30 to 0.45, the surface defects caused by vaporous emissions can be reduced and a desirable adhesion, particularly an especially desirable balance between porosity sealing and adhesion, can be further obtained. However, when the ratio UV/TH is within the above-mentioned ranges, the hardness and thus the scratch resistance of the coating composition may not be sufficient for practical applications.
Moreover, for easy processability, commonly used dual cure resins have a lower glass transition temperature, which is normally lower than 70° C. Although the resins with a lower glass transition temperature can be processes more easily, they exhibit poor heat resistance. Compared with the resins with a lower glass transition temperature, those with a higher glass transition temperature are more stable and more heat resistant, particularly when the resins are used in optical films or other components that will be affected by the light or heat from a lamp. When the temperature is up to 80° C., the form of the material is changed. Consequently, it is necessary to use the resins that have a higher glass transition temperature and a better heat resistance.